Satellite performance monitoring

ABSTRACT

Techniques for monitoring transmission performance of a satellite communications systems are provided, including techniques for measuring the primary contributors to the end-to-end SNR, including the uplink SNR, the downlink SNR, and the C/I for each link in the network. These individual measurements are used to estimate satellite effective isotropic radiated power (EIRP), satellite antenna gain-to-noise-temperature (G/T), and loss due to an Earth Terminal pointing error. The EIRP, satellite antenna G/T and loss due to Earth terminal pointing error may then be used to determine operating parameters for the satellite communications network that enable the network to operate more efficiently.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/024,522, titled “Satellite Performance MonitoringSystem”, filed Jan. 29, 2008, the content of which is herebyincorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to satellite communication systems and,particularly to satellite performance monitoring systems.

A hub-spoke, spot beam satellite system faces many impairments thatcontribute to the end-to-end signal to noise-plus-interference (SINR)ratio of a given link. The most common contributions to the end-to-endSINR include uplink signal-to-noise ratio (SNR), downlink SNR, andinternal system inference, such as that caused by other beams or otherusers in other beams. For example, internal system interference mayresult from other beams emanated from a multi-beam antenna of asatellite and/or may also result from interference from beamstransmitted from other Earth terminals.

In a typical satellite system design, no one term dominates the overalllink budget. From a perspective of network operation, configuration, andtrouble-shooting, having access to measurements of the primarycontributors to the end-to-end SINR, especially the uplink SNR, thedownlink SNR, and the carrier-to-interference ratio (C/I) would beuseful for optimizing the efficiency of the satellite communicationsnetwork.

Accordingly, techniques for determining the primary components of theend-to-end SINR for use in optimizing the configuration of a satellitecommunication system are desired.

BRIEF SUMMARY OF THE INVENTION

Techniques for monitoring transmission performance of a satellitecommunications systems are provided, including techniques for measuringthe primary contributors to the end-to-end SNR, including the uplinkSNR, the downlink SNR, and the C/I for each link in the network. Theseindividual measurements are used to estimate satellite effectiveisotropic radiated power (EIRP), satellite antennagain-to-noise-temperature (G/T), and loss due to an Earth Terminalpointing error. The EIRP, satellite antenna G/T and loss due to Earthterminal pointing error may then be used to determine operatingparameters for the satellite communications network that enable thenetwork to operate more efficiently.

According to an embodiment of the present invention, a method formonitoring the performance of a satellite communication system isprovided. The method includes measuring, at a gateway terminal, a totalreceive power of a signal received from a satellite under normaloperating conditions, and measuring a signal only power and/or a signalto noise plus interference ratio (SINR) for a signal received from thesatellite under normal operating conditions. The method furthercomprises measuring a total thermal noise component of the signalreceived from the satellite, measuring a downlink thermal noisecomponent of the signal received from the satellite, and calculatingsatellite communication system operating parameters using the totalreceive power, the signal only power and/or the SNR, the total thermalnoise component, and the downlink thermal noise component measurements.

Other features and advantages of the invention will be apparent in viewof the following detailed description and preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a satellite communications systemthat may be used with the techniques for monitoring performancedisclosed herein according to an embodiment of the present invention.

FIG. 2 is a high level flow diagram of a method for calibrating asatellite communications system according an embodiment of the presentinvention.

FIG. 3 is a high level flow diagram of a method for calibrating asatellite communications system according an embodiment of the presentinvention.

FIG. 4 is a high level flow diagram of method for determining terminalpointing loss using satellite EIRP.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an exemplary satellite communicationssystem 100 for which the performance may be monitored using thetechniques disclosed herein according to an embodiment of the presentinvention. Satellite communications system 100 includes a network 120,such as the Internet, interfaced with one or more gateway terminals 115that is configured to communicate with one or more user terminals 130,via a satellite 105. System 100 also includes one or more telemetry,tracking, and control (TTC) terminals 170.

Gateway terminal 115 is sometimes referred to as a hub, gatewayterminal, or ground station and services the uplink 135, downlink 140 toand from satellite 105. Although only one gateway terminal 115 is shown,embodiments of the present invention may have a plurality of gatewayterminals coupled to network 120. Gateway terminal 115 schedules trafficto the user terminals 130, although other embodiments could performscheduling in other parts of the satellite communication system 100.

A satellite communications system 100 applicable to various embodimentsof the invention is broadly set forth herein. In this embodiment, thereis a predetermined amount of frequency spectrum available fortransmission. The communication links between the gateway terminals 115and satellite 105 may use the same or overlapping frequency spectrumswith the communication links between satellite 105 and the userterminals 130 or could use different frequency spectrums.

Network 120 may be any type of network and can include, for example, theInternet, an IP network, an intranet, a wide-area network (WAN), alocal-area network (LAN), a virtual private network (VPN), a virtual LAN(VLAN), a fiber optical network, a hybrid fiber-coax network, a cablenetwork, the Public Switched Telephone Network (PSTN), the PublicSwitched Data Network (PSDN), a public land mobile network, and/or anyother type of network supporting data communication between devicesdescribed herein, in different embodiments. Network 120 may include bothwired and wireless connections, including optical links. As illustratedin a number of embodiments, the network may connect gateway terminal 115with other gateway terminals (not pictured), which are also incommunication with satellite 105. All gateway terminals in communicationwith satellite 105 may also connect with a command center 180.

Gateway terminal 115 provides an interface between network 120 andsatellite 105. gateway terminal 115 may be configured to receive dataand information directed to one or more user terminals 130, and canformat the data and information for delivery to the respectivedestination device via satellite 105. Similarly, gateway terminal 115may be configured to receive signals from satellite 105 (e.g., from oneor more user terminals 130) directed to a destination connected withnetwork 120, and can format the received signals for transmission withnetwork 120. Gateway terminal 115 may use a broadcast signal, with amodulation and coding (“modcode”) format adapted for each packet to thelink conditions of the terminal 130 or set of terminals 130 to which thepacket is directed.

Command center 180 connected to network 120 may communicate with eachgateway terminal 115 in the network and satellite 105. Gateway terminals115 may be generally located remote from the actual user terminals 130to enable frequency re-use.

Gateway terminal 115 may use an antenna 110 to transmit the uplinksignal to satellite 105. In one embodiment, antenna 110 comprises aparabolic reflector with high directivity in the direction of satellite105 and low directivity in other directions. Antenna 110 may comprise avariety of alternative configurations and include operating featuressuch as high isolation between orthogonal polarizations, high efficiencyin the operational frequency bands, and low noise.

In one embodiment of the present invention, a geostationary satellite105 is configured to receive the signals from the location of antenna110 and within the frequency spectrum transmitted. Satellite 105 may,for example, use a reflector antenna, lens antenna, phased arrayantenna, active antenna, or other mechanism known in the art forreception of such signals. The signals received from the gateway 115 areamplified with a low-noise amplifier (LNA) and then frequency convertedfor changing the power levels and frequencies. Satellite 105 may processthe signals received from the gateway 115 and forward the signal fromthe gateway 115 to one or more user terminals 130. In one embodiment ofthe present invention, the frequency-converted signals are passedthrough a bank of filters that separate the various frequency-convertedsignals having different bandwidth. A switch may select one of thevarious frequency-converted signals, which is then further amplified byTraveling Wave Tube Amplifiers (TWTA) to produce the desired EquivalentIsotropically Radiated Power (EIRP) at the payload antenna output. Thehigh-power transmission signal passed through a transmit reflectorantenna (e.g., a phased array antenna) that forms the transmissionradiation pattern (spot beam). In one embodiment of the presentinvention, satellite 105 may operate in a multiple spot-beam mode,transmitting a number of narrow beams each directed at a differentregion of the earth, allowing for segregating user terminals 130 intothe various narrow beams.

In some embodiments of the present invention, satellite 105 may beconfigured as a “bent pipe” satellite, wherein satellite 105 mayfrequency and polarization convert the received carrier signals beforeretransmitting these signals to their destination, but otherwise performlittle on the contents of the signals. A spot beam may use a singlecarrier, i.e., one frequency or a contiguous frequency range per beam. Avariety of physical layer transmission modulation and coding techniquesmay be used by satellite 105 in accordance with certain embodiments ofthe invention. Adaptive coding and modulation can be used in someembodiments of the present invention.

For other embodiments of the present invention, a number of networkarchitectures consisting of space and ground segments may be used, inwhich the space segment is one or more satellites while the groundsegment comprises of subscriber terminals, gateway terminals orgateways, network operations centers (NOCs) and a satellite and gatewayterminals command center. The gateway terminals and the satellites canbe connected via a mesh network or a star network, as evident to thoseskilled in the art. In one embodiment of the present invention, commandcenter 180 is connected to network 120 and is operative to transmitinstructions to the satellite and each participating gateway terminal inthe GSO communication system. In another embodiment, the command centermay be located at one geographical region and/or co-located with one ofthe gateway terminals 115. And yet in another embodiment, the commandcenter may be distributed amongst multiple geographical regions and/oramongst several gateway terminals. In yet another embodiment, thecommand center may be mobile and coupled to the network through acellular link or a wireless metropolitan (MAN) or a wide area network(WAN) link. The command center may be equipped with RF measurementequipment for measuring and evaluating interference characteristics.

The downlink signals may be transmitted from satellite 105 to one ormore user terminals 130 and received with the respective subscriberantenna 125. In one embodiment, the antenna 125 and terminal 130together comprise a very small aperture terminal (VSAT), with theantenna 125 measuring approximately 0.6 meter in diameter and havingapproximately 2 watts of power. In other embodiments, a variety of othertypes of antennas 125 may be used at the subscriber terminal 130 toreceive the signal from satellite 105. The link 150 from satellite 105to the user terminals 130 may be referred to hereinafter as the forwarddownlink 150. Each of the user terminals 130 may comprise a single userterminal or, alternatively, comprise a hub or router (not pictured) thatis coupled to multiple user terminals. In one embodiment, subscriberterminal 130 may comprise a receiver including a bandpass filter bankadapted to let through a GSO frequency spectrum and an extendedfrequency spectrum. Each subscriber terminal 130 may be connected tovarious consumer premises equipment (CPE) 160 comprising, for examplecomputers, local area networks, Internet appliances, wireless networks,etc.

TTC terminal 170 provides an interface for monitoring and controllingsatellite 105. For example, TTC terminal 170 may receive statusinformation from satellite 105, send commands to satellite 105, andtrack the position of satellite 105. In the present embodiment, TTCterminal 170 is connected to command center 180 via network 120, and TTCterminal 170 is configured to receive commands from command center 180and to send information, such as the status of satellite 105, to commandcenter 180. TTC terminal 170 may be an independent terminal, as shown inthe figure, or may alternatively be implemented in a terminal 115 thatalso carries traffic data.

According to some alternative embodiments, TTC terminal 170 may be indirect communication with command center 180 or may be integrated intocommand center 180, or may be integrated into gateway terminal 115. TTCterminal 170 communicates with satellite 105 using an antenna 175.Antenna 175 may be substantially similar to antenna 110 or may comprisea different configuration. Uplink 195 represents a command uplink fromTTC terminal 170 for sending commands to satellite 105. Downlink 190represents a telemetry downlink from satellite 105 for receiving datafrom satellite 105, such as data representing the position of satellite105. TTC terminal 170 may be located remote from gateway terminals 115and user terminals 130. These links may be in-band with the user datalinks 135 and 140, or alternatively use another set of frequencies.

According to embodiments of the present invention, a modem withingateway terminal 115 measures several operating parameters of satellitecommunication system 100 while satellite communications system 100 isoperating. These operating parameters include: (1) total receive power(S+N+I), (2) signal only power (S), and (3) end-to-end signal to noiseplus interference (SINR). The total receive power includes the signalpower (S) of a signal received from satellite 105, a total noise power(N) representing noise introduced by sources other than satellite 105,and interference power (I) representing the amount of co-channelinterference.

In order to determine the total noise power (N), all of the gatewayterminals are instructed not to transmit any energy to satellite 105.During the interval where there is no uplink energy reradiated bysatellite 105, the gateway terminal measures the total receive power,which will be N. For a typical GSO satellite, the interval where thereis no uplink energy reradiated by the satellite is approximately 0.25seconds after the gateway terminals and user terminals disable theiruplink transmissions). The value of N may be measured periodically byinstructing all of the gateway terminals 115 and user terminals 130 tonot transmit any energy to satellite 105.

Measurements are also made to determine the downlink thermal noise(N_(dl)) of gateway terminal 115. The measurement of the downlinkthermal noise is made while satellite 105 is essentially turned off or ameasurement is taken in a portion of the electromagnetic spectrum wherethe satellite does not radiate.

Using the results of these measurements, the gateway terminal 115 cansolve for the components of the link SINR for a link to satellite 105:uplink SNR, downlink SNR, and carrier to interference (C/I) ratio. Thelink propagation conditions in addition to gateway terminal 115'seffective isotropic radiated power (EIRP) and antennagain-to-noise-temperature (G/T), the satellite can determine the EIRPand G/T of satellite 105 in the direction of gateway terminal 115.According to an embodiment of the present invention, if a satellitecommunications system, such as satellite communications system 100,includes a plurality of gateway terminals each having known G/Tcharacteristics, the data can be jointly processed to determine anestimate of the pointing error loss for each gateway terminal.

Embodiments of the present invention may include measurements of some orall of the SINR components illustrated in the following equations:M1=S+N+I  (1)M2=S  (2)M3=S/(N+I)  (3)M4=N=N_(dl)+N_(ul)  (4)M5=N_(dl)  (5)Measurements M1, M2, and M3 may be made while the network is in normaloperating configuration. Measurements M1, M2, and M3 represent the powerat some common point in the receiver chain. According to someembodiments, measurements M1, M2, and M3 may be taken at the matchedfilter outputs (MFOs), which enables the digital signal processing to beused to perform measurements M1, M2, and M3. For example, according toan embodiment, measurement M1 may be performed by summing the square ofthe channel (in phase channel) MFO and the Q channel (quatraturechannel) MFO for N_(samp) samples and then dividing by N_(samp).

Measurement M2 may be obtained by various techniques known in the art.For example, according to some embodiments, M2 may be obtained throughtechniques that exploit the knowledge of certain symbols, such as uniquewords or pilot symbols, to estimate the power in the signal component ofthe matched filter outputs.

Measurement M3 may be obtained by various techniques known in the artfor estimating the SINR of a received digitally modulated symbol stream.In order to determine the components of the SINR, only one ofmeasurements M2 and M3 needs to be performed. Techniques for determiningthe other components of end-to-end SINR when either measurements M2 orM3 are available are described below.

Measurement M4 is made during a “quiet” interval where the uplinktransmissions from the plurality of Earth terminals, including gatewayterminals 115 and user terminals 130, have been disabled. According toan embodiment, measurement M4 is obtained one single hop delay after theuplink transmissions have been disabled. Measurement M4 involvesmeasuring the total received power and an identical process may be usedfor measuring the M4 value as the M1 value, except that transmissionsfrom the plurality of gateway terminals 115 and user terminals 130 aredisabled before measuring the M4 value. According to an embodiment,measurement M4 includes both the uplink thermal noise generated withinthe satellite, N_(ul), and the downlink thermal noise generated withingateway terminal 115, N_(dl).

Measurement M5 is made after satellite 105 has been commanded to disableall transmissions. For example, according to an embodiment, satellite105 may be commanded via an uplink command from command center 180 todisable all transmissions. Satellite 105 is commanded to enter into astate that prevents the re-radiation of uplink thermal noise. Uponentering the transmission disabled state, the resulting power receivedby gateway terminal 115 should be approximately 20 dB below the level ofthe thermal noise generated in gateway terminal 115 or lower. Accordingto an embodiment, this state may be achieved by switching in a largeattenuation value at some point prior to the satellite high poweramplifier (HPA). According to some embodiments of the present invention,uplinks from the plurality of gateway terminals 115 and user terminals130 may be disabled during this interval to minimize the “off” levelattenuation requirements on the satellite. Once the satellite (and thegateway and user terminals in some embodiments) have been placed in thedisabled state, the M5 measurement may be obtained using a processsimilar or identical to the process used to make measurement M1.Measurement M5 only includes the downlink thermal noise generated withingateway terminal 115, N_(dl).

According to an embodiment of the present invention where gatewayterminal 115 is configured to perform measurement M2 but not measurementM3, the components of the end-to-end SINR, the downlink SNR, the uplinkSNR, and the carrier to interference (C/I) ratio may be computed usingthe following equations:

$\begin{matrix}{\left( \frac{S}{N} \right)_{DL} = \frac{M\; 2}{M\; 5}} & (6) \\{\left( \frac{S}{N} \right)_{UL} = \frac{M\; 2}{{M\; 4} - {M\; 5}}} & (7) \\{\left( \frac{C}{I} \right) = \frac{M\; 2}{{M\; 1} - {M\; 2} - {M\; 4}}} & (8)\end{matrix}$Equation 6 represents the downlink SNR, Equation 7 represents the uplinkSNR, and Equation 8 represents the carrier to interference ratio.

According to an embodiment of the present invention where gatewayterminal 115 is configured to perform measurement M3 but not measurementM2, the components of the end-to-end SINR, the downlink SNR, the uplinkSNR, and the carrier to interference (C/I) ratio may be computed usingthe following equations:

$\begin{matrix}{\left( \frac{S}{N} \right)_{DL} = {\frac{M\; 1}{M\; 5}\frac{M\; 3}{{M\; 3} + 1}}} & (9) \\{\left( \frac{S}{N} \right)_{UL} = {\frac{M\; 1}{{M\; 4} - {M\; 5}}\frac{M\; 3}{{M\; 3} + 1}}} & (10) \\{\left( \frac{C}{I} \right) = \frac{M\;{3 \cdot M}\; 1}{{M\; 1} - {M\; 4\left( {1 + {M\; 3}} \right)}}} & (11)\end{matrix}$Equation 9 represents the downlink SNR, Equation 10 represents theuplink SNR, and Equation 11 represents the carrier to interferenceratio.

Measurements M1, M2, M4, and M5 are absolute power measurements (notratios like measurement M3) and are thus sensitive to absolute gain ofgateway terminal 115's front end. In order to minimize or eliminate thissensitivity, each of the measurements M1, M2, M4, and M5 should be takenwithin as short as short an interval as possible. According to anembodiment, measurements M1, M2, M4, and M5 should be taken while thefront end gain remains constant.

According to some embodiments of the present invention, only anoccasional snapshot of system performance is desired and thusmeasurements may be taken only occasionally to provide a snapshot of thesystems metrics for satellite communications system 100. According toother embodiments of the present invention, continuous monitoring ofsystems metrics is desired. In embodiments where continuous monitoringis desired, the measurements should be taken fast enough to trackchanges in the measurements.

For example, one of the quickest processes that may cause system metricsto change is rain fade. Rain fade refers to the absorption of asatellite signal by atmospheric rain, snow or ice, and may also refer toelectromagnetic interference at the leading edge of a storm front. Rainfade may occur due to precipitation at an uplink or downlink location asthe signal passes through precipitation en route to or from satellite105. According to an embodiment, measurements are taken approximatelyonce per second in order to track downlink rain fade events. Accordingto another embodiment, measurements are taken approximately once perminute, which should provide sufficient information for tracking rainfade events at a macro level and for tracking diurnal variations in thelink due to satellite movement.

According to some embodiments of the present invention, measurements M1,M2 and M3 may be taken continuously. However according to embodiments ofthe present invention, measurement M4 may be taken much less frequentlythan measurements M1, M2, and M3, because measurement M4 requires thatgateway terminal 115 and user terminal 130 transmissions be disabled.According to an embodiment, satellite system 100 comprises a widebandsystem where accurate power measurements for measurement M4 can beacquired very rapidly. For example, if the system is operating at 420million symbols per second (Msps), a 10 msec measurement interval wouldyield approximately 4 million symbols, which should be sufficient toprovide a good estimate of the sum of the uplink thermal noise and thedownlink thermal noise that comprises the M4 measurement. According tosome embodiments of the present invention, the M4 measurement is takenonce every second. If a 10 msec measurement interval is used, the outagetime required to take measurement M4 only comprises 1% of the operatingtime of satellite system 100. According other embodiments of the presentinvention, measurement M4 is taken once per minute, which in if a 10msec measurement interval is used, the outage time required to take themeasurement M4 only comprises 0.02% of the operating time of satellitesystem 100. According to some embodiments of the present invention, theoutage time used for taking measurement M4 is built into the framestructure so that the outage occurs at a regular interval. According toother embodiments of the present invention, the outage time used fortaking measurement M4 is periodically scheduled by a central controlagent, such as command center 180, by disseminating a message to allEarth terminals (including both gateway terminals and user terminals).

All gateway terminals 115 and user terminals 130 must be synchronized sothat their transmitters are disabled at precisely the same time in orderfor measurement M4 to be taken without any signal present. Inembodiments of the present invention that include return channelsemploying time division multiple access (TDMA), the user terminals (UTs)are already synchronized. For satellite communications, such assatellite communications system 100, that include gateway terminals(GWs), the gateway terminals may not include synchronizationfunctionality and the synchronization functionality must be added.

According to some embodiments, satellite communications systemarchitectures, synchronization of the GWs can be difficult when a GWcannot receive its own transmission, which would allow the GW to measurethe single hop satellite delay. One technique for synchronizing GWterminal transmissions, according to an embodiment of the presentinvention, is make a geometry-based calculation of the path delay basedon the fixed known locations of the GWs and based on satellite ephemerisdata, which may be easy obtained. The one way path delay combined withan accurate time base in each GW terminal can be used to synchronize GWterminal transmissions.

The M5 measurement involves sending a command to satellite 105 todisable all transmissions, which enables the total noise powermeasurement (N) to be divided into an uplink component (N_(UL)) and adownlink component (N_(DL)). Because this measurement M5 requiressatellite 105 to disable all transmissions, measurement M5 may beobtained less frequently than measurements M1, M2, and M3. According toan embodiment of the present invention, if the satellite EIRP (includingsatellite pointing error), the terminal G/T (including pointing error),and the propagation loss (including rain fade) has not changed since thelast time measurement M5 was taken, then the ratio of measurement M5 tomeasurement M4 will remain constant and can be expressed as the ratioM5/M4=γ. Thus, the value of M5 can be estimated using the followingequation:M5′=γM4  (12)wherein γ is the ratio of M5 to M4 at the last time that the M5measurement was performed in a no rain environment, M4 is the mostrecent M4 measurement, and M5′ is the estimated value of M5 that will beused instead of the M5 measurement. This technique for estimating thevalue of the M5 measurement may be used to significantly reduce thefrequency at which the M5 measurement need to be performed, therebyreducing the frequency at which satellite 105 must be instructed todisable all transmissions. This technique allows generation of accurateSNR and C/I estimates during intervals where there is no rain or otherprecipitation that could result in rain fade. Rain or otherprecipitation may result in errors in the SNR and C/I estimates. If thesatellite EIRP has varied since the last M5 measurement, the estimatedvalue of M5 will also be in error, resulting in errors in the SNR andC/I estimates.

According to another embodiment of the present invention, M5measurements are taken without sending a command to the satellite todisable all transmissions, enabling frequent M5 measurements to be takenwithout disrupting service on satellite communications system 100. M5measurements may be taken by measuring the noise power in a portion ofthe spectrum in which the satellite is not radiating any energy. Forexample, M5 measurements may be taken outside of the passband of thesatellite channel. This technique does not require that commands be sentto the satellite to change attenuators as in the other embodimentsdisclosed above, but gateway terminal 115 must be able to tune to aportion of the spectrum where the satellite does not radiate any power.Before using an M5 measurement made using this technique in equations(6)-(11), the measured out of band noise power should be adjusted by theratio of the in band measurement bandwidth (M4 measurement) to the outof band measurement bandwidth.

When using the out of band technique to make the M5 measurement, thegateway terminal may have a different system noise temperature and/orelectronic gain at the out of band frequency than at the in bandfrequency. These differences are primarily due to the frequency responseof the low noise amplifier (LNA), downconverter, modem, and cablingconnect the outdoor electronics, such as antenna 110, to the indoorelectronics, such as gateway terminal 115. In order for the M5measurement to be accurately determined using the out of band technique,the difference in noise levels between the out-of-band and the in-bandfrequencies must be known and a correction to the out of bandmeasurement applied. According to an embodiment of the presentinvention, M5 may be computed as M5=P5·Δ, where P5 represents theout-of-band power measurement and A represents the compensation factor.The compensation factor includes the known ratio of in-band noise levelto the out-of-band noise level as well as the ratio of the in-band toout-of-band measurement bandwidths.

The compensation factor Δ typically is not known and must be determinedby an gateway terminal, such as gateway terminal 115. According to anembodiment of the present invention, the following technique may be usedto determine Δ. This technique is typically applied during “scheduledmaintenance” periods. The technique includes periodically sending acommand to satellite 105 instructing the satellite to significantlyattenuate all transmissions for a short period of time. According to anembodiment, command center 180 sends the command to satellite 105 toattenuate all transmissions. According to some embodiments, thetransmission of the command to the satellite may be scheduled to occurat a predetermined interval. According to other embodiments of thepresent invention, the command may be issued to the satellite on an adhoc basis by command center 180. For example, the command may be issuedfor the purpose of measuring system metrics and/or for performingmaintenance on the satellite communications system.

According to some embodiments of the present invention, commands mayalso be sent to all gateway terminals 115 and user terminals 130 todisable uplink transmission from the gateway terminals. Disabling uplinktransmissions from gateway terminals 115 and user terminals 130 reducesthe level off attenuation requirements of satellite 105.

During the quiet interval resulting from the commands being issued tosatellite 105 and in some embodiments, to gateway terminals 115 and userterminals 130, all gateway terminals are configured to measure both thein-band noise power and the out-of-band noise power using the samemeasurement bandwidths that are used when regular M4 and M5 are made.The duration of the quiet interval should be sufficient to allowaccurate power measurements to be made. According to an embodiment ofthe present invention, the duration of the quiet interval should be lessthan one second.

After measuring the in-band noise power and the out-of-band noise power,each gateway terminal computes the ratio of the in-band noise power toout-of-band noise power to form the compensation factor Δ. The computedvalue of Δ is used for all M5 measurements until the next scheduledmaintenance period where the value of Δ is again determined.

The compensation factor Δ for each gateway terminal 115 is not expectedto change very much or very often, if ever at all. Thus, the “scheduledmaintenance” interval does not need to occur very frequently. Accordingto an embodiment of the present invention, a frequency of once a monthis sufficient to keep the compensation factor Δ updated and accurate.

As new gateway terminals 115 are added to satellite communicationssystem 100, the new terminals cannot participate in the SNR and C/Iestimation process until the new terminals have completed a Δcalculation in a scheduled maintenance interval. Thus, scheduledmaintenance intervals should be frequent enough to enable newlyprovisioned gateway terminals to participate in the SNR and C/Iestimation process. According to an embodiment of the present invention,a frequency of once a month is sufficient to enable newly provisionedgateway terminals to participate in the SNR and C/I estimation process.

FIG. 2 is a high level flow diagram of a method 200 for calibrating asatellite communications system using the techniques described aboveaccording an embodiment of the present invention. Method 200 usesin-band measurements to determine the value of measurement M5. Method200 begins with step 210 where the satellite communication system, suchas satellite communication system 100, is operated in a normal operatingconfiguration where the satellite and gateway terminal transmissions areenabled. At step 215, measurements M1, M2, and/or M3 (represented byequations (1)-(3)) are obtained using the techniques described abovewhile the satellite communications system is in the normal operatingconfiguration. At step 220, a command is sent to each of the pluralityof gateway terminals 115 and user terminals 130 to disable the uplinktransmissions from the gateway terminals and the user terminals. At step225, measurement M4 is obtained using the techniques described above.Measurement M4 (represented by equation (4)) includes both the uplinkthermal noise generated by satellite 105 and downlink thermal noisegenerated with an gateway terminal 115. According to an embodiment,measurement M4 is taken after a single hop delay after the uplinktransmission from gateway terminal 115 have been disabled. At step 230,gateway terminals 115 and user terminals 130 are instructed to return tonormal operating configuration. According to some embodiments, thecommands to the gateway terminals 115 and user terminals 130 to disableand reenable the uplink transmissions are provided to the gatewayterminals by command center 180 via network 120.

At step 235, a determination is made whether to perform the M5measurement or to determine an estimated M5 value based on the ratio ofa previous M5 value. Taking the M5 measurement involves sending acommand to satellite 105 to disable all transmission. Since thisdisrupts the operation of the satellite communications system, the M5measurement may be obtained much less frequently than measurements M1,M2, M3 and M4.

If measurement M5 is to be measured, method 200 proceeds to step 240,where the communications satellite 105, gateway terminals 115, and userterminals 130 are instructed to disable all transmissions. Aftersatellite 105, gateway terminals 115, and user terminals 130 have beeninstructed to enter this “quiet” mode, the M5 measurement is obtainedusing the techniques described above (step 245). After taking the M5measurement, the communications satellite and the Earth terminals arecommanded to return to normal operating configuration (step 247). Method200 then proceeds to step 255, where satellite system parameters arecalculated based on the values of measurements M1, M2 and/or M3, M4, andM5. According to other embodiments of the present invention, calculationstep 255 may be performed more frequently. For example, calculation step255 might also be performed after step 215 and/or step 230. Thesatellite system parameters may then be used to optimize the operatingconditions of the satellite broadcast system.

At step 235, if the M5 measurement is to be estimated, method 200proceeds to step 250, where an estimated value is determined for M5based on the value of M4 obtained in step 225 and a previouslydetermined value of M5 according to the techniques described above.After step 250, method 200 proceeds to step 250 where the satellitecommunication system is calibrated using the values of measurements M1,M2 and/or M3, M4, and the estimated value for measurement M5.

After step 255, method 200 proceeds to step 265 where a determination ismade whether to continue monitoring the operation of the satellitecommunication system. If additional monitoring of the operation of thesatellite communication system is to be performed, method 200 returns tostep 210. Otherwise, method 200 terminates.

FIG. 3 is a high level flow diagram of a method 300 for calibrating asatellite communications system using the techniques described aboveaccording another embodiment of the present invention. Method 300 usesan out-of-band technique for making M5 measurements that enablesfrequent M5 measurements to be taken without disrupting service on thesatellite communications system by measuring the noise power in aportion of the spectrum in which the satellite is not radiating anyenergy.

Method 300 begins with step 310 where the satellite communicationsystem, such as satellite communication system 100, is operated in anormal operating configuration where the satellite and gateway terminaltransmissions are enabled. At step 315, measurements M1, M2, and/or M3(represented by equations (1)-(3)) are obtained using the techniquesdescribed above while the satellite communications system is in thenormal operating configuration. At step 320, a command is sent to eachof the plurality of gateway terminals 115 and user terminals 130 todisable the uplink transmissions from the gateway terminals and the userterminals. At step 325, measurement M4 is obtained using the techniquesdescribed above. Measurement M4 (represented by equation (4)) includesboth the uplink thermal noise generated by satellite 105 and downlinkthermal noise generated with an gateway terminal 115. According to anembodiment, measurement M4 is taken after a single hop delay after theuplink transmission from gateway terminal 115 have been disabled. Atstep 330, gateway terminals 115 and user terminals 130 are instructed toreturn to normal operating configuration. According to an embodiment,the commands to the gateway terminals 115 and user terminals 130 todisable and reenable the uplink transmissions are provided to thegateway terminals by command center 180 via network 120.

M5 may be computed as M5=P5·Δ, where P5 represents the out-of-band powermeasurement and Δ represents the compensation factor. Determining thevalue of the compensation factor Δ requires that satellite 105 be putinto a “quiet” mode where all satellite transmission are disabled.Gateway terminals 115 and user terminals 130 may also be commanded todisable all transmissions while the compensation factor Δ is determined.Because this interrupts the operation of the satellite communicationsystem, the value of the compensation factor Δ typically will only bedetermined periodically and be used to estimate the value of M5 bymultiplying the compensation factor Δ by a current measurement of P5,which does not require that transmissions from satellite 105 beinterrupted. At step 335, a determination is made whether to determinethe compensation factor Δ or to determine an estimated value for M5value based on a previously obtained compensation factor Δ.

If compensation factor Δ is to be determined, method 300 proceeds tostep 340, where the communications satellite 105 is instructed todisable all transmissions. Gateway terminals 115 and user terminals 130may also be commanded to disable all transmissions. After satellite 105,gateway terminals 115, and user terminals 130 have been instructed toenter this “quiet” mode, in-band and out-of-band power measurements aretaken by an gateway terminal according to the techniques described above(step 345). After taking the in-band and out-of-band power measurements,the communications satellite and the gateway and user terminals arecommanded to return to normal operating configuration (step 347). Method300 then proceeds to step 349, where the compensation factor Δ isdetermined using the techniques described above. After determining thecompensation factor Δ, method 300 continues with step 355, where anestimated of measurement M5 is determined using the techniques describedabove. After completing step 355, method 300 proceeds to step 370 wheresatellite system parameters are calculated based on the values ofmeasurements M1, M2 and/or M3, M4, and M5. According to otherembodiments of the present invention, calculation step 370 may beperformed more frequently. For example, calculation step 370 might alsobe performed after step 315 and/or step 330. The satellite systemparameters may then be used to optimize the operating conditions of thesatellite broadcast system.

At step 335, if the M5 measurement is to be estimated using a previouslydetermined compensation factor Δ, method 300 proceeds to step 350, wherean out-of-band power measurement is obtained while satellite 105 isoperating in a normal operating mode. Method 300 then proceeds to stepwhere an estimate of measurement M5 is determined using the previouslydetermined compensation factor Δ and the out-of-band power measurementobtained in step 350 using the techniques described above. Method 300then proceeds to step 370.

After step 370, method 300 proceeds to step 375 where a determination ismade whether to continue monitoring the operation of the satellitecommunication system. If additional monitoring of the operation of thesatellite communication system is to be performed, method 300 returns tostep 310. Otherwise, method 300 terminates.

Determining Satellite EIRP and Terminal Pointing Loss

Equation 13 illustrates the relationship between the downlink SNR andthe satellite EIRP:

$\begin{matrix}{\left( \frac{S}{N} \right)_{DL} = {\frac{{EIRP}_{sat}}{L_{p}}\left( \frac{G}{T} \right)_{ET}\frac{1}{k}\frac{1}{R_{sym}}}} & (13)\end{matrix}$In equation (13), R_(sym) represents the symbol rate of the link, whichmay be different for the forward link and for the return link, krepresents Boltzmann's constant, (G/T)_(ET) represents the G/T of thegateway terminal including any pointing loss, EIRP_(sat) at representsthe satellite EIRP allocated to the symbol stream and in the directionof the gateway terminal, and L_(p) represents the propagation lossincluding any rain loss. Equation (14) is derived from equation (13) bytaking 10 times the Log of equation (13) and rearranging the results:

$\begin{matrix}\begin{matrix}{{\left\lbrack {EIRP}_{sat} \right\rbrack + \left\lbrack \left( \frac{G}{T} \right)_{ET} \right\rbrack} = {\left\lbrack R_{sym} \right\rbrack + \lbrack k\rbrack + \left\lbrack L_{p} \right\rbrack + \left\lbrack \left( \frac{S}{N} \right)_{DL} \right\rbrack}} \\{{= \left\lbrack {C(n)} \right\rbrack},{1 \leq n \leq N}}\end{matrix} & (14)\end{matrix}$where the notation [x] represents 10 Log 10(x). The right-hand side ofequation (14) consists of known quantities when downlink SNR determinedusing equations (6) or (9) is used. Hence, equation (14) may be used toaccurately determine the sum of the gateway terminal G/T (in dB/K) andthe satellite EIRP (in dBW) when it is not raining. According to anembodiment, for return links, the gateway terminal is a gatewayterminal, which are typically carefully installed and have an accurateauto track pointing system, and thus have a well-known G/T. Thus, thesatellite EIRP is easily derived using the technique describe above.

For the forward link, the gateway terminal is a user terminal that doesnot have an auto tracking point system and may not have a non-trivialamount of pointing error. However, the boresight G/T of the userterminal is generally fairly accurately known. According to anembodiment the present invention, the satellite EIRP and pointing lossfor each user terminal may be derived by examining data from many userterminals in locations where the satellite EIRP is expected to beapproximately the same value. According to an embodiment of the presentinvention, equation (15) illustrates a technique for determining thepointing loss of user terminals. Data is collected from N identical userterminals at locations with identical satellite EIRP values and the G/Twith pointing loss may be expressed as a boresight G/T plus a pointingloss term according to equation (15):

$\begin{matrix}{{{\left\lbrack {EIRP}_{sat} \right\rbrack + \left\lbrack \left( \frac{G}{T} \right)_{ideal} \right\rbrack - \left\lbrack {L_{point}(n)} \right\rbrack} = \left\lbrack {C(n)} \right\rbrack},{1 \leq n \leq N}} & (15)\end{matrix}$where [L_(point)(n)] represents the pointing loss of the n^(th) userterminal of N user terminals, where N is a positive integer greater than1, and [C(n)] is the constant from the right-hand side of equation (14)using the downlink SNR calculation for the n_(th) user terminal.EIRP_(sat) is unknown but is constant for all n, (G/T)_(ideal)represents the boresight G/T of the user terminal and is known andconstant, L_(point)(n) is not known, and C(n) is known from the downlinkS/N calculation for all n user terminals.

When surveying a large population of user terminals, at least one of theuser terminals is likely to have a very small and insignificant pointingloss. According to an embodiment of the present invention, by using theuser terminal SNR measurement that results in the largest value of[C(n)] in equation (15) with an assumed zero pointing loss error, thevalue of EIRP_(sat) can be calculated. The pointing loss for the otherterminals may then be calculated using equation (15) using thedetermined value of EIRP_(sat).

FIG. 4 is a high level flow diagram of method 400 for determiningterminal pointing loss using satellite EIRP using the techniquesdescribed above according to an embodiment of the present invention.Method 400 begins with step 410, where the value of C(n) is determinedfor each of N user terminals. At step 415, a maximum value of C(n) isdetermined from the values of C(n) determined in step 410. This valuewill be used as a “gold standard” for determining the pointing losses ofthe other user terminals. In step 420, the EIRP of the satellite isdetermined with the assumption that the pointing loss of the n*th userterminal is zero (the n*th user terminal represents the user terminalhaving the largest value of C(n) determined in step 415). Once the EIRPof the satellite has been determined, the pointing loss of each of theother user terminals of the N user terminals may be determined (step425). After completing step 425, method 400 terminates.

Determining Satellite G/T

The uplink SNR is related to the gateway terminal EIRP and satelliteG/T. According to an embodiment of the present invention, equation (16)illustrates the relationship between the uplink SNR, the gatewayterminal EIRP, and satellite G/T:

$\begin{matrix}{\left( \frac{S}{N} \right)_{UL} = {\frac{{EIRP}_{ET}}{L_{p}}\left( \frac{G}{T} \right)_{Sat}\frac{1}{k}\frac{1}{R_{sym}}}} & (16)\end{matrix}$

Using the calculated uplink SNR, the sum of the user EIRP and thesatellite G/T can be determined using the equation (17):

$\begin{matrix}{{\left\lbrack {EIRP}_{ET} \right\rbrack + \left\lbrack \left( \frac{G}{T} \right)_{Sat} \right\rbrack} = {\left\lbrack R_{sym} \right\rbrack + \lbrack k\rbrack + \left\lbrack L_{p} \right\rbrack + \left\lbrack \left( \frac{S}{N} \right)_{UL} \right\rbrack}} & (17)\end{matrix}$where EIRP_(ET) represents the EIRP in the direction of the satelliteand includes pointing loss, G/T_(sat) represents the EIRP in thedirection of the gateway terminal, and L_(p) represents the propagationloss including any rain loss. In equation (17), the right-hand side ofthe equation consists of quantities that are known (when there is norain loss) and the left-hand side of the equation consists of quantitiesthat may not be known.

For the return channel, the EIRP_(ET) corresponds to the gatewayterminal. A gateway terminal generally has the high power amplifier(HPA) output coupled to a power meter, so the HPA output is accuratelyknown. Antenna gain is also generally accurately known and an autotrackpointing system employed to keep the pointing error very small. Hence,the gateway terminal EIRP is accurately known, thereby enabling thecalculation of the satellite G/T in the direction of the gateway.

The EIRP is generally not accurately known for low cost user terminals,since a power meter is typically not included as part of low cost userterminals. However, if the satellite G/T is accurately known (or isassumed), then equation (17) may be used to determine the EIRP of eachuser terminal. However, if the satellite EIRP is not accurately known,the user terminal EIRP and the satellite G/T cannot be determined.

According to an embodiment of the present invention, one solution tothis problem is to employ one or more “advanced” user terminals ordiagnostic terminals within the beam. The advanced terminals shouldinclude both accurate power measurement capability and accurate pointingcapability. An accurate estimate of the EIRP for the advanced terminalcan be determined by collection of the power meter measurement for theterminal. This value can then be used with equation (17) to determinethe satellite G/T. Once the satellite G/T has been determined, the EIRPfor all other user terminals can then be determined.

The disclosure provides various techniques for monitoring theperformance of a satellite communication system including measuring theprimary contributors to the end-to-end SNR, such as the uplink SNR, thedownlink SNR, and the C/I for each link in the network. Techniques forderiving the estimate satellite effective isotropic radiated power(EIRP), satellite antenna gain-to-noise-temperature (G/T), and loss dueto an gateway terminal pointing error from the uplink SNR, the downlinkSNR, and the C/I for each link in the network are also provided. Thevarious performance measurements derived using these techniques may thenbe used to optimize the configuration of the satellite network toprovide optimal throughput. The various measurement obtained through thetechniques described above may be used to configure the satellite systemto provide optimal throughput.

While the embodiments described above may make reference to specifichardware components, those skilled in the art will appreciate thatdifferent combinations of hardware and/or software components may alsobe used and that particular operations described as being implemented inhardware might also be implemented in software or vice versa.

Thus, although the invention has been described with respect to specificembodiments, it will be appreciated that the invention is intended tocover all modifications and equivalents within the scope of thefollowing claims.

What is claimed is:
 1. A method used on a satellite communicationsystem, wherein the satellite communications system includes asatellite, at least one gateway terminal and a plurality of userterminals, the method comprising: measuring, at the at least one gatewayterminal, a total receive power of a signal received from the satelliteunder normal operating conditions; measuring one of a signal-only powerand a signal-to-noise-plus-interference ratio (S1NR) for a signalreceived from the satellite under normal operating conditions; measuringa total thermal noise component of the signal received from thesatellite during a first quiet interval where uplink transmission fromthe at least one gateway and the plurality of user terminals have beendisabled; measuring a first out-of-band downlink thermal noise componentand a first in-band downlink thermal noise component while the satelliteis in a transmission disabled state; determining a compensation factorusing the first in-band downlink thermal noise component and the firstout-of-band downlink thermal noise component; measuring a secondout-of-band downlink thermal noise component while the satellite isunder normal operating conditions; estimating a second in-band downlinkthermal noise component of the signal received from the satellite bymultiplying the second out-of-band downlink thermal noise component bythe compensation factor; and calculating satellite communication systemoperating parameters during satellite operation using the total receivepower, the signal-only power or the SINR, measurements of the totalthermal noise component, and the second in-band downlink thermal noisecomponent for monitoring the performance of the satellite communicationsystem in which the satellite is operating.
 2. The method of claim 1further comprising: commanding the at least one gateway terminal and theplurality of user terminals to disable all transmissions prior tomeasuring the total thermal noise component of the signal received fromthe satellite; and commanding the at least one gateway terminal and theplurality of user terminals to enable transmissions after measuring thetotal thermal noise component of the signal received from the satellite.3. The method of claim 2 wherein an outage time used for measuring thetotal thermal noise component of the signal received from the satelliteis built into a frame structure of the satellite communications systemso that the outage time occurs at a regular interval.
 4. The method ofclaim 2, further comprising: scheduling an outage time for measuring thetotal thermal noise component of the signal received from the satellite,wherein the outage time is scheduled by a central control agent of thesatellite broadcast system.
 5. The method of claim 4 wherein schedulingan outage time for measuring the total thermal noise component of thesignal received from the satellite further comprises: commanding the atleast one gateway terminal and the plurality of user terminals todisable all transmissions for an interval specified in a message.
 6. Themethod of claim 2 wherein commanding the at least one gateway terminalto disable all transmissions prior to measuring the total thermal noisecomponent of the signal received from the satellite further comprises:calculating a path delay to the satellite from the at least one gatewayterminal based on the location of the at least one gateway terminal andephemeris data of the satellite; and synchronously disabling the atleast one gateway terminal with the disabling of the plurality of theuser terminals using the path delay and an accurate time base of the atleast one gateway terminal.
 7. The method of claim 1, furthercomprising: commanding the satellite to disable all transmissions priorto measuring the first in-band downlink thermal noise component and thefirst out-of-band downlink thermal noise component; and commanding thesatellite to enable transmissions after measuring the first in-banddownlink thermal noise component and the first out-of-band downlinkthermal noise component.
 8. The method of claim 1 further comprising:commanding the satellite, the at least one gateway terminal, and theplurality of user terminals to disable all transmissions prior tomeasuring the first in-band downlink thermal noise component and thefirst out-of-band downlink thermal noise component; and commanding thesatellite, the at least one gateway terminal, and the plurality of userterminals to enable transmissions after measuring the first in-banddownlink thermal noise component and the first out-of-band downlinkthermal noise component.
 9. The method of claim 1 wherein both thesignal only power of the signal received from the satellite under normaloperating conditions and the SINR for the signal received from thesatellite under normal operating conditions are measured.